Magnetic film storage device with nondestructive readout



Sp- 99 s. WDDELHQEK 465,635

MAGNETIC FILM STORAGE DEVICE WITH NNDESTRUCTIVE READOUT SINON HIDELHOEKATTORNEY Sept. 9, W6@ s. MIDDELHCEK MAGNETIC FILM STORAGE DEVICE WITHNONDESTRUCTIVE READOUT 7 Sheets-Shee Filed Feb. l5, 1966 M W M v i l l iI l I l i I l I l i I I I I l il* jvz vi L Q M E H l i l I i I I i l l Il I l I I I Il.. O m 4++ f` M 4 .LA 111111111111111111111 I1 al Md 2J,L. a uw. M a ha a mu um. H ML Vim v M m w M M HM M 1J i@ 1J T l I I I lI l I i 111W l I I l I Il l ++4+++ 2 2 0 x .7 Tha hw Tm M Mh UWM Ww UWMM M hv, m: v? M il 1. l l H M H M m C D A B C D B S. MIDDELHCEK Sept 9,E969 MAGNETIC FILM STORAGE DEVICE WITH NONDBSTRUCTIVE READOUT 7Sheets-Sheet Filed Feb. l5, 1966 ATV Sem 9, w69 s. MaDDELHoEK 3,456,635

MAGNETIC FILM STORAGE DEVICE WITH NONDESTRUCTIVEI READOUT Filed Feb. l5,1966 '7 Sheets-Sheet 4 Sept. 9, w59 s. MIDDELHOEK 3,455,535 MAGNETICFILM STORAGE DEVICE WITH NONDESTRUCTIVE READOUT med Feb. 15. 196e 7Sheets-Sheet 5 s. MlDDr-:LHQEK 3,466,635

MAGNETIC FILM STORAGE DEVICE WITH NODESTEUCTIVE READOU'X Sept. 9, 3%9

7 Sheets-Sheet 7 Filed Feb. l5, 1966 US. Cl. 340-174 3 Claims ABSTRACTOF THE DHSCLOSURE Nondestructive readout is provided by means of amagnetic film storage element which is divided into alternately arrangedhard and soft zones wherein the magnetization vectors normally arepointed in the same direction along an easy axis that. extendstransversely through the boundaries of all the zones. When this storageelement is subjected to a read field applied substantially along thehard axis of the film, the vectors in the soft zones readily' rotateinto or toward their hardaxis positions, whereas the vectors in the hardzones rotate only slightly or negligibly by comparison so that the hardzones serve to restore the magnetizations of all zones when the readfield terminates.

This invention relates to data storage cells of the magnetic film type,particularly those which are adapted to operate in a nondcstructivereadout, orthogonal switching mode.

Magnetic tilm cells for storage of binary information presently arebeing used in storugc devices of data processing systems. Theanisotropic lms used have a preferred axis of magnetization, theeso-called easy axis. whereby the magnetization of a film can assumeeither of two stable positions for selectively storing the binary values"l" and "0." The stored information can beread out by applying magneticfields that cause a rotation or reversal of the film magnetization. Whenmagnetic fields arc applied in the direction orthogonal to the preferredaxis. i.c., in the s0-called hard direction, extremely rapid (of theorder of nanoseconds) coherent rotational switching results, whilefields applied parallel to the preferred axis cause a significantlyslower (of the order of microseconds) wall switching and are thusunsuitable for use in data stores operating at extremely high speeds. Inmost storage devices presently being used the stored informaf tionnormally is destroyed by the readout operation,

There also have been developed magnetic film cells and modes ofoperation thereof that permit nondcstructive readout of the storedinformation. In some prior devices of this nature there are two filmsarranged above or beside each other whose easy axes are parallel, andtwo states of the films each characterized by the antiparallelpositioning Ot' the respective film magnetizations are used forinformation storage. The magnetization of one film. called the switchingfilm" or "read film," is rotatively switched into the hard direction bya read pulse fcd to a word liuc running parallel to the easy axis of thefilms, Such switching produces :in output pulse in a sense line runningothogonally to the word line. The second or magnetically harder" film,which will be called tilt iid-@5,535 i Patented Sept. 9, i969 thestorage film," can be switched only by applying fields that are strongeror of longer duration than the read pulse, and it is only slightlydeflected from the easy axis by a read pulse. The two films are soarranged that after the word pulse has ended, the magnetization of theread film is switched back into the direction antiparallel to themagnetization of the storage film by reason of the stray-held couplingbetween the lilins. This orientation of the magnetizations correspondsto the binary value that had been stored before the readout operation;hence, the operation is nondestructve.

A magnetic coupling of two films which tends to align the magnetizatiousof the films in antiparallel directions is called "negative" coupling.Double film cells with negative coupling have thc disadvantage that thewrite operations, in which the magnetizations of the two films may haveto bc switched and aligned in antipai'allel directions, require eitherrelatively complicated write pulse sequences with elaborate circuitryand long write times, or an elaborate construction of the memory cellitself (a line being arranged between the two films). Strong strayeldcoupling automatically' causes the antiparallel alignment of themagnetizations` but since this orientation is accomplished by wallswitching. the switching takes too much time for high-speed storagedevices. Furthermore. in using double film cells with negative coupling.the Sense signal is decreased in that, when the transverse read field isapplied, the magnetization of the storage film is also slightlydeflected from the easy direction. so that a voltage is induced in thesense line, arranged parallel to the hard direction. whose polarity isopposite to that of the voltage generated by the switching of the readfilm.

Read-only stores are known in which permanent magnets associated withindividual magnetic film cells determine preferred directions ofmagnetization. each parab lel to one of thc two easy directions, thatcorrespond to the binary values to be stored :ind to which themagnetization returns after the rotation caused by application of a readheld; whereby nondcstructive readout is obtained. These arrangementshave the disadvantage that the fields specifying the stored informationcannot be reversed electrically, which would he desirable for theconvenient write-itt of new information. There also are known certainkinds of magnetic film cells in which thin films of diflerentcoercivities are arranged one above the other and are separated by rtnonmagnetic metallic layer. in which magnetic coupling tends to alignthe magnetizations of the films parallel to each other in the samedirection. Coupling of the kind which causes the respectivemagnetizations of the films to be parallel tie.. pointing in the samedirection) commonly is described as Vpositive coupling," and in thoseinstances where the positively coupled films are arranged in superposedrelationship` it also may be referred to as exchange coupling. Thereason for the positive coupling achieved in this latter arrangement isnot completely understood. lt is assumed that it is caused either bydiffusion of small particles of magnetic material into the nonniagncticmetal layer or by the fiction of the electrons in said layer.

An object of thc present invention is to improve thc construction andoperation of magnetic film cells, particularly those of thenondestructive readout (NDRO) type, by employing the positive couplingprinciple to gain the attendant advzinagcs of simple :ind economicalfab` 'a e) rication. short-duration operating cycles and comparativeinsensitivity' to disturbances.

A further object is to provide an improved NDRO storage device whichenables the stored information to be readily changed by means of simpleinput circuitry.

A general object is to improve the techniques for nialting magnetic filmcells, especiallyl those ofthe NDR() type, so :is to avoid thelimitations of prior cells.

'l`o attain the above-stated objects, it is herein proposed to fabricatea magnetic film in a manner .sticn that it has interspersed zones ofdifferent magnetic properties arranged .side-by-.side along a commoneasy axis extendingy across the one boundaries. so that .said zones arepositively coupled to one another and differ from each other in theirrotational responses to transversely applied cx citations. wherebydifferent magnetic field strengths are required to rotate the respectivemagnetizations of the yones toward the hard direction.

The foregoing and other objects. features and advantages of theinvention vtill be apparent from the tol lovving more particulardescription of preferred embodiments ofthe inventionI as illustrated inthe accompanying drawings.

In the drawings:

FIG la is a schematic representation of a conventional single-layer thinmagnetic film cell in conjunction with its input and output means.

FIG. lh is the critical curve of such a cell.

FIG. 2u is a section through a film consisting of yones having identicaldimensions but different magnetic properties in accordance with theinvention.

FIG. 2l) is a diagram illustrating the behavior of the magneti/ation andcoupling held vectors in the various '/.oncs of such a film.

FIG. Zt' is a diagram illustrating the magnetic flux present in thezones of said film.

FIGS. 3a. 3b and 3c are diagrams of the magnetization components 5f.. inthe easy direction vs. the magnetic field components IIX acting in thedirection of the easy axis, for:

tal a conventional single film,

(bl a multiwne film with IBX HC7 tcl a multizone film with HBY IC- FIG.4u is a section through a film consisting of zones of differentthicknesses having different magnetic properties` in accordance with theinvention.

FIG. 1li is a diagram illustrating the behavior of the magnetization andcoupling field vectors in the film of FIG. 4a.

FIG. 4." is a diagram illustrating the magnetic llux present in the7tnes of the film shown in FIG. du.

FIG. 5 shows the critical curves of the .switching and .storage zones ofa multizone hlm where the zones are of idcntigal dimensions. as in FIG.2u.

FIG. 6 shows the critical curves of the switching and storage zones of aniulti7one film where the` zones are of different dimensions, as in FIG.111.

FIG. 7u is a section through an embodiment of a magnetic film cellconstructed according to the invention.

FIG. 7h is a plan view of said embodiment.

FIG. tiri is a schematic representation of a NDR() storage array builtaccording to the invention.

FIG. fili is a diagram showing the behavior, during a read operation, ofthe field and niagneti7ation vectors antl the resulting .sense signalsfor stored "Il" and stored 1" magnetic` film cell of the kind utilii'edin the array of FIG. Sil.

FIGS. 0u. l/n Illa, ttl/v, ilu and It/i are sectional and plan viewsshoning .still other enilotlinients of magnetic film cells' according:to tht` invepion l`lt`ts`. lu and l/i illustrate tht` construction andswitchin;y behavior of conventional single-layer thin magneti: filmcells in a magnetic film memory operated in the solll Ill)

.fill

called orthogonal field driving mode. FIG. lil shows a thin magneticfilm cell tt) whose easy axis Rr, is parallel to the .v-tlircctiondetermined by the input and output means. As is; generally lvnotvn. inthe orthogonal field mode the magnetiation of the film is rotated duringreadout and write-in at least approximately into the direc tion of thehard axis RH by a word field HV acting in the 'vf-direction, which isgenerated by a pulse fed to word line l2 by word driver II, and whoseamplitude is; larger tlian the saturation field .strength I'IK (FIG.lli). The film is thus` magnetically saturated in its` hard-axisdirection. 'lite voltage induced during readout in .sense line I3 andpassed to sense amplifier I-t is proportional to the change in themagnetization component in the .r-tlirection, ic., (IMX/tlf, and itspolarity is characteristic for the` information stored in the cell.During write-in, an additional bit field applied orthogonally to theword field. ic., in the .v-direction, and generated by a pulse fed tobit line 16 by hit driver I5, determines by its polarity the binaryvalue ("0" or "1") to be written in. The axes ofthe word. bit, and senselines, usually designed as strip lines. define an orthogonal .system ofcoordinates whose r-drection ideally lies parallel to the easy axis RLof the niagrictic anisotropy of the film.

FIG. lh shows the .so-called critical curve 17, an asteroid. which, asis ltnown. defines' the magnetic switching behavior of a single-domainstructure. ln the rotational switching processes to beconsideredsinglcdomain behavior can be assumed for the magnetic filmsused. so that the asteriod thus also applies' to these switching films.'lhe .raxis of the asteroid corresponds to the easy axis RL of the film.and orthogonal to RI, is the hard axis RH.

Normally the stable .states characterized by the alignment of themagnetization in one of the two directions of the easy axis are used forinformation storage. This is shown in FIG. Ib by the two arrows It) and19 designated "O" and l." Rotational .switching of the magnetization canbe effected by magnetic fields larger than the values defined by theasteroid. For example. a field applied in the hard direction must exceedthe value III.: to make such switching possible. An additional fieldcomponent in the direction of the easy axis is needed to causeunequivocal switching into one of the desired stable positions. Theselatter held components can be generated in different ways, e.g., by abit pulse fed to bit line 16. or, as will be described in more detail.by the magnetic coupling between different lnis or zones of a thinmaenetic film cell. h

In the following description there first will be presented some basicexplanations, illustrated by FIGS. 2a through 6, which are considerednecessary for an :idequate understanding of the positive magneticcoupling principle which determines the operation of the variousmagnetic film cells` embodying the invention. FIG. 2n shows the partialcross .section of a film 20 consisting of magnetizable material ofuniform thickness. The film 2f) can consist, for example. of tflfi Niand Ztlfi Fe, and its thickness can be approximately 500 A. I.et it beassumed that it is an anisotropic film and that the .section throughthis film is parallel to the assumed easy-axis thereof. Ict it beassumed further that the film 20 consists of several interspersedstrip-shaped Zones 23 and 24 extending parallel to the hard asis of thehlm and differing in their respective I'IK values. the hard" zones 23having a higher Il and the "soft" zones 24 a lower IIK. FIG. 2/1 ist adiagrammatic plan view of part of this filni under various conditions--(The behavior of the film at its edges will not be considered in detailsince this is not necessary for an understanding of the operationalmode.) When no external field acts on the hlm, the inaunt-tiyations ofthtA Itard t.\f) and ilu' soft (,vftl /ont's are in thc position sho-vinin section of the film, ic., parallel to the casu' diiecfion l v.Assuming that for all lones the peoructiic dimensions. width andthickness',

'aie identical, and that the llitv. density lt is equal, then since lM112M1, the respective components Mx of the inagnetic flux, FIG. 2c, arealso equal. (aMxzO, as indicated in section A of FIG. 2c.)

Sections B of FIGS. 2b and 2c show the conditions existing when anexternal field Hy is applied in the hard direction, where Hy is largerthan the HK value of the soft zones 24 (HKI) but significantly smallerthan that of the hard zones 23 (Hgh). The intluenceron the magnetizationof the hard zones is neglected 'for the time being. The magnetizationvector M1 rotates into the position designated M1', forming an angle pwith the easy direction. The component M1Y parallelto the hard axis doesnot contribute to the magnetic coupling of the zones. It merelyinfluences the conditions at' the film edges lying parallel to the easyaxis and therefore will not be considered here. The component M1X in theeasy direction is smaller by the factor cos p(M1 \,=M1-cos p) thanMhJzMh). The flux differences designated in section B of FIG. 2c withAM,c and emphasized by 4,- and symbols thus result at the zoneboundaries. The resulting magnetic field lines fromftto pass partlythrough the air adjoining the film and through the ground plate (notshown), and partly through the film itself. The resultant fields thusproduced in the film that affect the direction of the magnetization areshown in FIG. 2b by the coupling fields +HB (in the soft zones) and thedemagnetizing fields HB (in the hard zones).

Section C of FIGS. 211 and 2c again show the conditions occurring whenan external field Hy is applied. In contrast to the assumption made forsections B, that the magnetization M1, remains unaffected by field Hy,the deflection of M1, is here taken into account. It is assumed that thehard zones 23 show the same switching behavior (i.e., coherentrotational switching when Hy HKh) as do the soft zones 24. Thedeflection of tlie magnetization M11 by an angle d caused by field Hyresults in a decrease of the r-component from M1, to M11x=M1fcos 1L. Theincremental magnetism AMX, and accordingly the field strength HB, arethus decreased.

Section D of FIGS. 2b and 2c show the magnetizations and fields whenmagnetizations M11 and M1 are in antiparallcl positions in the absenceof an external field. This state is unstable when the coupling field-t-HBX exceeds the coercivity HC of the soft zones, as indicated in FIG.3c.

The curves representing the magnetization components Mx vs. the fieldstrength Hx that are obtained when external clds arc applied in thedirection of the easy axis are shown for different films in FIGS. 3athrough 3c. FIG. 3a shows curve 30 for a conventional homogeneous film.The intersections with the H,l axis are symmetrical with respect to theM, axis. This means that the field strength Hx required for switchingthe magnetization of such a film is approximately equal to thecoercivity HC of the film material, both in switching from state 1 to 0"and in switching from 0 to "1.

FIG. 3b shows the corresponding Mx=f(ffx) curve 31 for the soft zones 24of the film of FIG. 2a. Here the fields applied must be so chosen thatno switching takes place in the hard zones 23. The magnetization M1, ofthe hard zones is aligned in the -1-x direction. Owing to the effect ofthe coupling field HBK, the curve is asymmetrical with respect to theM,c axis. A preferred direction results for magnetization M1. The fieldstrength Hx required for switching from 1" to 0" is HXIHC-i-Ilm, i.e.,greater than that required for switching from O to l, which isHXIHC-lfm. Since IIC II11X, the two positions l and 0 are stable.

FIG. 3c again shows the Mx==fflx) curve 32 for the soft zones 24 of thefilm of FIG. 2a. In contrast to thc case illustrated in FIG. 3b, it ishere assumed that IIC H11U so that the position designated becomesunstable.

FIGS. 4a through 4c, like FIGS. 2a through 2c, show the structure of afilm 40 with zones of different HK values, as well as the vectors of theniagnctizations and the fields. In the structure shown here, however,cach of the hard zones is composed of a strip-like portion d3 of thefilm 40 with a second film 42 placed on top thereof, so that the hardzones 42413 are thicker than the intervening soft zones 44. As sectionsA of FIGS. 4b and 4c show, magnetic flux differences Occur at 'the zoneboundaries even when no external field Hy is applied. This results incoupling field strengths-HB0, to which are added the fields HB when afield Hy is applied, as is shown in sectionsl B. Similar conditionsexist when the magnetic flux of the hard and soft zones is different. Ifthe width of the hard and soft zones is different and the thicknessequal, then HB0 becomes zero; but the value of the coupling anddemagnetizing field strengths +HB and -HB. respectively will differ.

FIG. 5 shows, for the film of FIG. 2a, the critical curves for the softzones 44 (curve S6), henceforth called switching zones, and for the hardzones Lf2-43 (curve 51), called storage zones. As has been describedhereinabove, owing to the coupling fields there is a preferred directionfor the magnetization M1 of the switching zones 44, i.e., parallel tothe magnetization M11 of the storage zones. The two stable states M11and M1 in direction -1-x and M11 and M1 in direction -x can therefore beused for storing binary information` as indicated in the figure by 1"and "0. Let it be assumed that the magnetization vectors are at first inthe position designated as "1. When a field of the order HKh Hy HK1 isapplied (where HKh and HK] are the H1; values of the storage and theswitching zones, respectively) magnetization M1 will be rotated at leastapproximately into the hard direction by angle p, while magnetizationsM1l are deflected only by the small angle 1.0. lt is here assumed thatthe material permits coherent rotational switching of the storage zones.Otherwise, unless the applied fields cause wall switching, the effect onthe magnetization of the storage zones will be negligible. Owing to themagnetic coupling, coupling or demagnetizing fields +HB or -HB act inaddition to field Hy, the resulting fields atecting the magnetizationbeing designated H11 and H1. The resulting directions of magnetizationsM11 and M1 are obtained in the known manner by drawing the tangents fromthe ends of the field vectors H1, and H1 to the corresponding asteroid(curve Slt or lf the coupling field -t-HB acting in the -t-xdirectionwere not present, the magnetizations M1 of the switching zones would beentirely rotated into the hard direction (-l-y). after the Hy field hadterminated. there would thus be no field causing the magnetizutions M1of all domains of the switching zones to return into either the -tx orthe x-direction, so that antiparallel splitting of the domains wouldresult. In the film shown in FIG. 2a, however, magnctizations M1 returninto their original position, since they had not been entirely rotatedinto -the hard direction and so fall back into the closer easydirection, and since they are influenced by the coupling fields +1 H11,which become zero only after complete rettirn of the magnctizations intothe.

easy direction. M1, also returns to the easy direction.

This analysis shows that the coupling field strength H13 present inaccordance with the stored binary value ensures nondcstructive readoutof the stored information,

since the magnetization of the film cell is returned into its originalposition after each readout operation.

FIG. 6 shows, for the structure of FIG. 4a. the critical Curves for thesoftswitching zones 44 (curve 60) and for the hard storage zones 42-43(curve (il), and it illtistrates the effect of the coupling fields H111,which are shown in FIG. 411.

FIG. 7a shows a thin magnetic film ccll embodying the invention. A Nidclayer (80% Ni, 20% Ve) is deposited onto a polished ground plate 7lconsisting of nonmagnctizable material, the film thickness beingapproximately 50() and the coercivity IIC of the film materialapproximately 3.5 oe. Strips 72 of a cobaltnicltel film (60G: Co, 40%Ni) are directly evaporated onto tite r\'i-l `e film 7l) through ascreen, the coercivity of these strips 72 being considerably higher thanthat of the Ni-l"e film` 70, erg., IIL-cl3 oe. The HK values of the tuohlnis are in about the saine ratio as their llc values'. i.e.. the HKvalue of the CoANi filtri is considerably higher than that of the Ni-lefilm. Exemplary diniensioiis of the (li-Ni filin 72 are: thickness 50()A.; strip width and distance between strips, both about SUO/4. Sinceboth the Ni-l`e and Co-r 'i films are very thin and are in directContact with one another, there is strong exchange coupling between themwhich prevents inded pendent rotation Of the magnetization in only oneOf the superposed film layers. As a result. the zones 73 of the Ni-lchlm 70 situated directly below the Co-Ni strips 72 also becomemagnetically hard. i.c.. the effective HK and HC values increase inthese zones 73. Film 7). then, consists of zones 74 with relatively lowHC and'llK values and intervening zones 73 with relatively high HC andHK values, whereby zones 74 and 73 can be used as switching zones andstorage zones, respectively.

When a field Hx is applied in the direction of the easy axis RL. herethe maximum value of HX is greater than the HC value of the switchingzones 74 and smaller than that of the storage zones 72-73. a curve.llu'xzftI-Ix) corresponding to that of FlG. 3b results. The directionof the field strength HBY determining the preferred direction depends onthe orientation of the magnetization in the storage zones` which is notswitched by the field applied. The field strength HB measured for thethin I.

magnetic film cell described is 1.5 oe. It is easy to show that it ispositive couplingwhich is involved ie., that the magnetization of theswitching zones 74 tends to align itself in a direction parallel to thatof the storage zones 72-73. This is illustrated in FlG. 7b by the arrows75 and 76 respectively representing the magnetizations in the varioussoft and hard Zones.

When there is applied in the hard direction a field Hy whose value liesbetween the HK values of the storage and the sstitching zones, themagnetization of the switching zones 74 will be rotated approximatelyinto the hard direction (perpendicular to RL), while that of the storagezones 72-73 iiill be only slightly deflected. The thin magnett; filrncell shown in FIGS. 7a and 7h has basically the saine behavior as thatof the cells illustrated in FIGS. 2u and 4./1.

Note that when the thin magnetic film cell described is used in a storehaving an orthogonal driving mode. differerit HK values of zones 73 and"f4 are essential for the required formation of storage and switchingzones. whereasdifferent HC values, although advantageous, are notessential,

FIGS. Sri and 8h illustrate an embodiment of a NDRO storage arrayemploying the magnetic film cell of the present invention. `Nhen such anarray is operated as a read-only store (ROSl. only read and no writeoperations talte place. At the outset. however. it is necessary that themagnetization of thc storage cells be aligned in accordance with thebinary values which are to be stored and later read out. This: write-inoperation can be achieved electrically by applying sufficient strongfields fll.. llv.{, 'l'he required input circuits, being of an obviousdesign, are not shown.

l`lG. Rrr schematically shows the matrix arrangement of the store.Storage matrix S0 consists of thin magnetic filrn cells 8l, cach havingthe properties of the inventive cell described hereinabove. which arearranged in rows and columns. Word lines ft2. parallel to the easydirection ltr, of the cells` arid scuse lines 83, orthogonal thereto,are arranged above these storage cells. 'l'he orthogonal :irrartgeuiciitof word rind sense lines reduces inductive coitplimz between theselilies` and thus the coupled noise signals, to a iiriiiirnum. 'flicstore is word-organized; that is. in each read operation the bits ofbinary information .stored iti a number of cells constituting a word:irc icad ffl out simultaneously. the readout itt this instance beingnondestructive. As was described for FIG. 5, this requires. for eachcell. the application of a field in the hard direction of approximatelythe value llKl. This field is generated by pulses fed to selected wordlines tiZ by word drivers 84. these word drivers being .selected foroperation in accordance with the address of the word to be read out. lf.for example, the word driver 8f4-2 feeds a pulse to the word line til-2,then the inagnetizations M1 (FIG. Sfr) in the switching zones of thestorage cells associated with that word line are rotated approximatelyinto the hard direction. 'l'lie change of the magnetization componentlying in the easy direction (x-direetion) that occurs in cach cellassociated with the word to be read out induces pulses in the resrective sense lines 83 arranged above these cells, which pulses areamplified by sense amplifiers 85 respectively associated with thesenselines. During a readout operation cach sense amplifier thus receives apulse or pulse sequence whose polarity or phase is characteristic of thebinary value ("l" or (V) in the corresponding bit position ofthe wordread out.

FIG. 8b shows the magnetic fields acting on the switching zones of thecells during a readout operation. as well as the resulting rotations ofthe magnetization vectors of the storage zones (Mh) and the Aswitchzones (hill. This figure is based on the assumption that a stored binary"U" is characterized by the alignment of the inagnetizations Mh rind M1in the -,vdirection and that the binary l corresponds to their alignmentin the --v-direction. The coupling fields influencing the magnetizationof the switching zones in the .v and the +A=directions are designatedl-[B-- and HB--lu respectively. and the word field generated by the wordpulse is designated H5.. H1 is the resulting effective field for tlteswitching zones of the film. Curve 36 represents aniplitude-timevariation of the word pulse fed to one of the word lines 82. Curves B7and 88 respectively4 represent on a time scale the voltages induced in.sense lines 83 for a "0 and a l stored in the associated storage cell.This otitptit voltage is led to a discriminator circuit (not shown) todetermine the binary value of the bit which was read out, according tothe polarity or phase of the voltage.

The read-only store just described cart be expanded or modified into ascmipermanent type of nondestruetive readout store-4.o.. :i store withhigh-speed readout capability whose storcd information can be changed bylower-speed write operations-hy providing permanent writewriteincircuitry which permits switching the storage zones of the thin magneticfilm cells as desired. Since the write-in of new information is doneonly occasionally. it need not be pcrforiired at high speed. Thenecessary switching circuits therefore are inexpensive. even when commonbit-sense lines cannot be utili/.ed for both writing and reading. Forusing these cells in a type of nondcstructive readout (NDRO) storewherein the write-in operations mtist also be performed very rapidly. itis necessary to decrease the Hl.; andw HC values of tht` storage zonesof the thin filtri cells. Since a certain ratio of the HK and l-lCvalues of the switching and .storage zones must be maintained. very lowl-lK and HC values become necessary' for' the switching zones feg..11K-:0.5 oe., IIC-.0.2 oel. Methods are known. however. for obtainingsuch values.

NGS. 911 through lllr illustrate .several other magnetic film cellsembodying the invention. ln thc embodiment shown in lilflS. 911 and 9/1.a Ni-fI film 9() about 400 A. thick is deposited oirlo a polished groundplate fil. The cocrcivity of the film material is 3.0 oe. Si() films 9S,which :ire about 5() X 50p. iti dimension. are deposited ihiour'li ascreen onto the film 9i). 'l'liesc Si() squares 95 :rie arianr'ed inrows and i'olurirris on the film ltl at :t distance of about Slip fioriicach other, l'ilrri l2 consists of (`o-i\li and has a coeicivity of I3oe., heini'` about dll() A. thick. fri tht '/'oncs 93 of film 9() wherethere is direct contact between the bfi-lie filrn Ill and the (follifilm 92, the HK and HC values increase owing lo the cxchange-couplingeffcct of the magnetically hard Cri-Ni lilni 92. The SiO film spots 95magnetically decouple the films 92 and 90 so that the zones 94 of theNilie film 90 covered by the Si() film spots 95 are practicallyunaffected by the hard film 92. ln this embodiment the film 90 (like thefilm 70 in the thin film cell illustrated in FIG. 7a) has zones withdifferent HC and HK values. Zones y93, FIG. 9a. can serve as storagezones and zones 9d as switching zones. As in the foregoing embodiments,there is positive coupling between the storage and switching zones. Thefield strength HEX (FlG. 3b) is 2.5 oe. l

ln the embodiment shown in FIGS. 10a and 10b, thin strip-shaped silverlayers M52 about 200 A. thick are evaporated through a screen onto apolished ground plate 101. A Ni-Fe film 100 about 500 A. thick with acoercivity of 1.5 oe. then is deposited on the plate i101, covering thesilver strips 102. ln zones 103 the Ni-Fe film i042 and the ground plate101 are separated by the silver layer E02, which is so thin that it doesnot form a continuous layer but consists of a number of microseopicallysmall islands. Thus. the Ni-Fe film 100, in effect, is vapordepositedonto a surface roughened by the silver at 102, and since the thicknessof the Ni-Fe film 1GO (which also is relatively thin) is approximatelyconstant, both the contact surface and the upper surface of the Ni-l-`efilmare uneven in the contact zones 103. The stray magnetic fields thatresult from this roughening of the surfaces cause an increase in the HCvalue of the film i90. An HC of oe. has been measured in a device ofthis kind. The silver here serves only for roughening the surface. Suchroughening can be accomplished in other ways, eg., by etching ormechanically abrading the surface of the plate 01 in the desired places.Owing to the higher HC, the direction of the molecular magnetizationdepart further from the measurable direction of the total magnetizationof the film, which remains unchanged. This entails an increase in theheld strength required to rotate the entire magnetization into the harddirection. Macroscopically, such a film corresponds (in its zones 03) toa film with high HK. In zones 104 the NiFe film 100 is in direct contactwith the polished surface of ground plate fill, and here the magneticproperties of the film remain unchanged. In this embodiment, therefore,the film (like the film '/'0 of the thin film cell illustrated in FIG.7a) has zones of different magnetic properties. Zones 103 can serve asstorage zones, and zones Mld as switching zones. There is positivecoupling between these zones, the field strength being measured as 1.25oc.

ln the embodiment shown in FIGS. lla and 1lb, a

continuous Ni-Fe film about 50() A. thick is deposited onto a polishedground plate lll. Thin strip-shaped copper layers i12 are evaporatedonto film 110 through a screen. A temperature increase after depositioncauses diffusion of the copper 112 into the Ni-Fe film H0, resulting ina local increase of the HC value in the zones 113 of film 110 andthereby an increase in the field strength required for rotating themagnetization of the material into the hard direction in those zones.The unaffected zones 11dof the Ni-Fe film llt) are interspersed with theZones 113. ln this embodiment, film llt? (like film 70 of the thin filmcell illustrated in FIG. 7a) has zones of different magnetic properties.Zones i131 can serve as storage zones and zones 11d as switching zones.It will be noted that there is positive coupling between these differentzones as in the case of the previously described embodiments.

The invention has been described with reference to several preferredembodiments. lt should be noted that the structures, dimensions, andmaterials selected herein for illustration are merely representative.Thin film cells having other values (e.g., of the coupling fieldstrength), appropriate for a given application, can be produced bychanging the relevant dimensions (filtri thickness, width, length; orthe placing of the film spots deposited through "rrlsogmtliy'fil'msdescribed as consisting of 80% yiand `gopperr1102251..,fstliadilfus@than be Si a screen) and by the choice of licand HK values of the films used. ln producing thcsc thin film cells, forexample, the processes described in the following references can be usedto obtain the desired llc and HK values:

l-lc value: Nature, 194 (1962), page 1035, and IBM Research Report RZ154 (1964).

HI.; value: Proceedings of the lntermag Conference, 1964, chapter 9.3.

In the case of the embodiments illustrated in FIGS. l0allb, thehardening materials mentioned above, eg., the silYLQllYilldoiottghcn thground plate and the 20% Fe, and of 60% Co and 40% Ni, can be ofdifferent coriipositions.

While a read-only store (FIG. 8a) was chosen as an embodiment in whichthe magnetic film cells of the invention can advantageously be used,other storage arrangements, such as stores with high-speed read andwrite operations, can also be devised to utilize cells according to theinvention which merely have different values than those in the examplesdescribed.

All of the embodiments disclosed herein are characterized by the featurethat their storage and switching zones are positively coupled together,thereby eliminating the disadvantages of negatively coupled films asmentioned hereinabove. lt should be noted also that where storage andswitching zones are provided by the presence or absence in such zones ofhardening means (such as overlying strips of high-anisotropy material,or diffused hardening agents, or microscopic protuberances on thesubstrate) in accordance with the invention, the extent of suchhardening effects is accurately controlled and precisely selectedbeforehand, so that the various zones are geometrically well dehned in apredetermined pattern rather than being disposed merely by chance in arandom arrangement of hard and soft spots throughout the cell. This is adecided advantage over prior magnetic film ceilS in which positivecoupling and interruptions of such coupling are randomly distributed inan uncontrolled manner within each cell.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

What is claimed is:

il. A magnetic film cell for use in :i data store of the type whichoperates in an orthogonal switching mode, said cell comprising:

a layer of niagnetiznble material having a rst axis along which themagnetization of said layer normally is directed and having a secondaxis transverse to said first axis along which said layer can bemagnetized temporarily to rotate the magnetization thereof away fromsaid first axis toward said second axis,

said layer having a plurality of distinct zones arv ranged side by sidein a predetermined manner along said first axis so that the boundarybetween cach adjacent pair of said zones is crossed by said first axis,said zones being magnetically coupled in such a way that theirrespective magnetizations tend to be aligned parallel with said firstaxis and uniformly directed; and hardening means intimately associatedwith said layer of magnetizable material in at least a prcdtermined oneof said zones but not in any of said zones immediately adjoining such aprcdcteimincd zone for thereby causing the rnagnclivahle material insaid predetermined zone or zones to havcmagnelic properties differentfrom those ofl any said adjoining zone, whereby the several zones differin the transverse magnetic field Strength required to rotate theirrespective magnetizations away from said lirat artis.

2. A magnetic film eell according to elaim i wherein mid hardening meanscomprises a second layer of magnetizable material having an .'iniaotropyvalue higher than that of 'the material contained in the tiret saidlayer, aid two layers heini; in an exeltzliigecotinled relationxhip witheach other in only that predetermined zone 0r Zones u herein themagnetic held strength required to rotate the magneti/,ation thereof isSelected to exceed the magnetic held strength whieh is required torotate the magneti/.tb tion of an adjoining zone or zones.

3. A magnetic hlm morage deviee with nondestruetive readout comprising:

a magnetic lilm cell of the ltind specified in claim i',

and

reading means for momentarily applying to all zones of said eell amagnetic held directed substantially alongy said second axis and beingof such magnitude L and duration that it rotates through approximatelyninety degrees the magnetization of eaeh of the unhardened zones,without rotatingy by :my comparable amount the magneti/dion of any ofthe hardened zones, whereby Said hardened zones serve to restore themarcznetifations of all zones to their original direction along saidfirst axis when said magnetic field terminates.

References Cited TlRRlll. W. FEARS, Primary Examiner BARRY L. HALEY,Assistant Examiner

